1. (a) Technical Field
The present invention relates to a system and method for controlling a fuel cell system. More particularly, the present invention relates to a system and method for controlling a fuel cell system which minimizes excessive supply of air that is unintended and unnecessary and thus reduces unnecessary energy consumption for an operation of an air blower and improves fuel efficiency of a fuel cell vehicle as a result.
2. (b) Background Art
A fuel cell system as applied to a hydrogen fuel cell vehicle typically includes a fuel cell stack for generating electricity via an electrochemical reaction between reactant gases, a fuel processing system (FPS) for supplying hydrogen as a fuel to the fuel cell stack, an air processing system (APS) for supplying oxygen-containing air as an oxidant required for the electrochemical reaction in the fuel cell stack, and a thermal management system (TMS) for removing reaction heat from the fuel cell stack to outside of the fuel cell system, controlling an operating temperature of the fuel cell stack, and performing water management.
In the air processing system, external dry air is forcibly blown by an air blower into a membrane humidifier and, at the same time, supersaturated moist air discharged from a cathode outlet of the fuel cell stack is fed into the membrane humidifier. At this time, the dry air is humidified by water exchange between the dry air and the moist air, and the humidified air is supplied to the cathode inlet of the fuel cell stack.
At present, the air is supplied to the cathode of the fuel cell stack is about two times the stoichiometric ratio (SR. The amount of air supplied to the cathode affects the output of the fuel cell stack, the efficiency of the fuel cell system, the relative humidity of air, the water balance, etc. In particular, depending on the amount of air, when the operating temperature is low, such as during start-up or warm-up of the fuel cell system, flooding (excessive condensation) may occur. Furthermore, when the operating temperature increases, such as during high power operation, a polymer electrolyte membrane my dry out in the fuel cell stack. Thus, it is very important to optimally control the amount of air supplied to the cathode to improve the performance of the fuel cell.
As a conventional method for controlling the amount of air supplied, a method of calculating a target flow rate based on information related to the current of the fuel cell stack using an air stoichiometric ratio has been used. To control the amount of air supplied to the fuel cell stack, a controller calculates the target flow rate, calculates an RPM command value of the air blower such that the air is supplied to the fuel cell stack at the calculated target flow rate, and controls the RPM of the air blower to follow the calculated RPM command value. However, excess air may still be supplied to the cathode in this control method.
In particular, there is a difference between an SR calculated under actual operating conditions of the vehicle and a target SR as shown in FIG. 1. When the SR calculated under actual operating conditions is higher than the target SR, more than an optimum amount of air is supplied to the fuel cell stack (i.e., excessive supply of air). This means that the air blower is being operated more than is necessary to reach a target value, which consumes much more energy than necessary for the operation of the air blower, thus reducing the fuel efficiency of the fuel cell vehicle.
Moreover, during the operation of the air blower, significant noise is generated, and the water balance in the fuel cell stack is affected due to the unnecessary excessive supply of air, which may possibly cause the polymer electrolyte membrane to dry out, thus reducing the performance of the fuel cell.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.